The present invention relates to fiber optic gyroscopes (hereinafter referred to as “FOG”), and more particularly, to FOG coils constructed and arranged to reduce the rate of absorption of ambient moisture.
It is well known that moisture can degrade the performance and reliability of optical fibers. Micro-cracks in the glass fiber can propagate in the presence of ambient moisture which in turn can change the optical properties of the fiber and potentially lead to premature failure. The effects of moisture depends on many factors, including environmental conditions, the nature of the fiber manufacturing process, etc. Since the amount of moisture can change with environmental conditions (e.g., temperature), the optical properties of the fiber can change, often unpredictably, as a function of those conditions.
Fiber manufacturers typically apply an acrylate (or other similar polymeric material) protective coating directly to the outer surface of the glass fiber to mitigate the effects of ambient moisture. Such a coating creates a barrier to moisture, provides some level of abrasion resistance and permits handling since bare fiber is very fragile. While the coating may environmentally protect the fiber, the coating itself may absorb a significant amount of moisture. This phenomenon has been observed during bake-out procedures, i.e., when the fiber is subjected to controlled high temperature environments, for an extended amount of time. During bake-out procedures, coated optical fibers experience a significant weight change (e.g., 12 percent or more), and hence a diametrical change, implying that the coating surrenders a significant amount of captured moisture while in the high temperature environment.
A coil of optical fiber is a critical component in an Interferometric Fiber Optic Gyroscope (IFOG, or more simply, FOG). A FOG is a device used to measure the rate of rotation of a vehicle or other platform to which the FOG is attached. The FOG typically includes a coil of optical fiber disposed about an axis of rotation. A light source transmits light into each end of the optical fiber, so that two light transmissions propagate through the optical fiber in counter-rotating directions. Detection circuitry receives the light transmissions as they emerge from the ends of the optical fiber and measures the relative phase relationship of the light. The phase relationship of the two light transmissions is related to the angular rotation of the FOG coil about the axis of rotation, and may be used to derive an output signal that is indicative of the rate of rotation of the FOG coil.
An important parameter associated with a FOG, commonly referred to as the “scale factor,” defines and quantifies the relationship between the actual rate of rotation of the FOG to the output signal of the FOG device (e.g., number of output pulses per arc-second of rotation). Variations in the FOG scale factor tend to decrease the accuracy of the FOG. The optical diameter of the fiber optic coil directly influences the scale factor of the FOG, so any external influences that could affect the optical diameter will also affect the scale factor. The optical diameter is closely related to the physical diameter of the coil, so any change in the physical diameter of the coil can effect the scale factor of the FOG. Therefore, moisture absorption by the fiber jacket directly affects the overall fiber diameter and hence the resultant scale factor of the FOG.
Epoxy materials are often applied about and between layers of optical fibers in the coils to provide physical stability of the winding layers, and to maintain the coil geometry over environmental stresses. Such epoxy materials are known to be amorphous with inhomogeneities that are commensurate with the size of water molecules, so as to permit the transport of water molecules through capillary action. These epoxy materials are thus hygroscopic, and if the stabilizing epoxy material absorbs a significant amount of moisture, the epoxy material can expand and/or deform, thus changing the coil geometry and affecting the performance of the FOG.